Production of inorganically colored coatings on aluminum

ABSTRACT

In procedure and apparatus where inorganically colored coatings on aluminum are produced by first anodizing the aluminum articles and then treating such anodically coated aluminum with alternating current in an acidic bath containing metal ions for producing a colored deposit in the coating, the alternating current treatment is effected between an anodized aluminum workload and a plurality of electrode elements constituting a counterelectrode, which are distributed along a region that faces the workload, e.g. elements of the same metal as the ions in the bath, the distribution and spacing of the elements being of a described nature such as to improve the desired results and to facilitate control. With substantially uniform alternating current density over the anodized aluminum in the above manner, control of operation is further facilitated by maintaining a selected current value condition or conditions for corresponding current density control, preferably such that the color obtained is determinable in accordance with duration of the treatment.

Montreal, Quebec, Canada [54] PRODUCTION OF INORGANICALLY COLORED COATINGS ON ALUMINUM 15 Claims, 8 Drawing Figs.

[52] U.S. Cl 204/35 N, 204/42, 204/288, 204/DIG. 7 [51 Int. Cl C23b 5/50, C23b 5/68 [50] Field ofSearch 204/38.l,

DIG. 7, 285, 35 N, 42, 286, 288, 289

OTHER REFERENCES Brenner, A., Electrodeposition of Alloys Vol. 1 Academic Press, New York, 1963 pp. 165-168.

Graham, A. K., Electroplating Engineering Handbook Reinhold Pub. Co., New York, 1962, pp. 487- 488.

Lowenheim, F. A., Modern Electroplating, John Wiley & Sons Inc, New York, 1963, pp. 26 27.

Primary Examiner-John H. Mack Assistant Examiner-41. J. Fay

AnorneysRobert S. Dunham, P. E. Henninger, Lester W.

Clark, Gerald W. Griffin, Thomas F. Moran, R. Bradlee Boal and Christopher C. Dunham ABSTRACT: In procedure and apparatus where inorganically colored coatings on aluminum are produced by first anodizing the aluminum articles and then treating such anodically coated aluminum with alternating current in an acidic bath containing metal ions for producing a colored deposit in the coating, the alternating current treatment is effected between an anodized aluminum workload and a plurality of electrode elements constituting a counterelectrode, which are distributed along a region that faces the workload, e.g. elements of the same metal as the ions in the bath, the distribution and spacing of the elements being ofa described nature such as to improve the desired results and to facilitate control. With substantially uniform alternating current density over the anodized aluminum in the above manner, control of operation is further facilitated by maintaining a selected current value condition or conditions for corresponding current density control, preferably such that the color obtained is determinable in accordance with duration of the treatment.

CONSTANT Cl/PPf/VI' CONT/POL PRODUCTION OF INORGANICALLY COLORED COATINGS ON ALUMINUM BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to procedure and apparatus for producing inorganically colored coatings on aluminum sur-, faces, i.e. articles of aluminum, such term including aluminum base alloys and thus generally signifying aluminum that is appropriate for anodic treatment. More particularly the invention is concerned with operations such as described in US. Pat. No. 3,382,l60, granted May 7, 1968 to Tahei Asada. According to the patent, inorganically colored coatings are produced by first anodizing an aluminum article, as in sulfuric acid solution, to form an anodic coating, and then subjecting the anodized article to electrolytic treatment with alternating current in an acidic bath containing metal ions selected from the group consisting of the following cations and anions: Ni, Co, F e, Cu, Agfi Cd, Zn, Pb, and anions consisting of oxygen combined with one of the metals Se, Te and Mn. The patent further states that by the described process there is deposited in the anodic coating the oxide or hydroxide of metal of the selected ions, i.e. such metal in chemical combination with oxygen, resulting in a colored coating which can be sealed and which has good permanence.

The present improvements are specifically related to the step of electrolytic treatment with alternating current passing between the anodized work and a couterelectrode, e.g. a metal electrode, whereby a colored deposit is effected in the anodic coating, resulting from metal of the selected ions, and important aims of the improved method and apparatus are to promote achievement of selected colors or tones, without defects and in a reproducible manner, and to attain a new and simplified mode of control for applying any desired shade of a given color or metal oxide deposit.

2. Description of the Prior Art The process to which the above-cited US. Pat. No. 3,382,I60 relates has been found highly effective for its intended purpose and has been put to extensive use for treating aluminum articles to achieve a variety of colors or tones for instance identified as bronze, brown, red-brown, gold, grey, black and in most instances a number of shades from pale to dark in each case. The use of nickel ions in the bath, with the alternating current passing between the work and a nickel electrode, has been quite successful, attaining a number of shades of bronze that are in demand for architectural and other purposes, and notable results have been achieved with copper-containing baths to yield pale reds, dark red-brown, and darker colors. Other particularly suitable operations have related to one or another of the metals silver, selenium and tellurium, the latter two being employed in the form of anions, whereas silver, nickel and copper appear in the bath as cations.

For a number of reasons including the popularity of bronze tones, alternating current baths containing nickel ions have been most widely used, but considerable potentiality also exists for other metals of the group, notably copper. Some difficulty has been encountered with darker shades of color, for instance in that upon treating aluminum articles with nickel ions in the acidic bath, attainment of dark bronze colors is sometimes or often accompanied by so-called spalling, namely the appearance of minute light or white specks or spots, where it would appear that the colored coating has fallen away, leaving these small, usually round, bright patches.

Another difficulty that sometimes arises is in reproducibility, i.e. in matching a color or shade obtained on a work load with the selected color or shade of a previous workload, and especially so where the workloads are of different sizes, even though care is exercised in selection of voltage or of a program of voltages and in the time or times of treatment. Problems of this sort have occurred in various baths such as nickel and copper, and can be a matter of concern where the user of colored aluminum requires a large number of pieces or articles to be identical in appearance. Requisite control has sometimes been found very delicate, notably in making voltage adjustment in the case of copper; for example in utilizing a bath containing copper ions and passing AC between the anodized work and a copper sheet electrode, it has been difficult even to obtain a medium shade, e.g. maroon or the like, as distinguished from a pale tone or a near-black or black.

A primary object of the present invention is to improve the operation in one or more or indeed all of these respects, so as to facilitate the attainment of desired colors, in a consistent manner. Another significant object is to provide improvement in control, and in a different manner than by the selection or adjustment of both the voltage of the applied alternating current and the time of treatment as heretofore practiced. the ef fect of the improved control being to obviate or minimize situations of poor matching or nonreproducibility where such may tend to occur, and also, in presently preferred operation, to permit achievement of a selected shade of color essentially only in accordance with duration of treatment.

SUMMARY OF THE INVENTION To the above and other ends it has now been discovered that advantageous results, especially in the reproducible, satisfactory attainment of desired shades, are achieved in the color-depositing step, by directing the flow of alternating current from and to counterelectrode surfaces, as of suitable metal, that consist in effect of a multiplicity of separate elements, and indeed for special advantage constitute essentially widely spaced elements (more generally, widely spaced relative to their useful areas), e.g. arranged in an appropriate array facing the aluminum workload under treatment, so that in the preferred, most economical arrangement the actual effective, current-passing area of the counterelectrode is only a minor fraction of the total area over which such electrode elements are distributed or the corresponding total external area of the anodized workload surface that more or less faces the counterelectrode (or is affected by flow of current to and from the electrode) and that is to be colored. Upon operation in this manner, for example with the actual total, electrically effective area of counterelectrode elements being, say, from about 3 to about 25 percent of, or preferably at least substantially less than the area over which the facing surfaces of the workload and electrode are distributed, tendency to spalling, for instance with nickel baths, can be essentially obviated in circumstances where it may otherwise exist, as where a dark bronze or like shade is sought. Moreover, the reproducibility of the operation or the availability of control to attain selected tones or shades, is in general improved for various baths, whether containing nickel, copper or other selected metal, and indeed the present process markedly facilitates attainment of intermediate colors as in the case of copper.

In practice, the electrode assembly, which may appropriately be made of the same metal as the selected ions in the bath, can be fashioned in various ways to afford the spaced, distributed structure. A convenient arrangement consists of an array of parallel, vertical strips, rods or tubes of the stated metal, or like pieces such as the connected elements of a continuous grid, screen or the like, disposed in the tank so as to define a selected vertical plane and having relatively wide spacing, i.e. a distance between successive pieces which may be as much as several times the transverse dimension of the effective, current-passing surface of each piece, conveniently at least four times and most preferably more. With such assembly, the passage of alternating current, through a suitably selected bath, between the counterelectrode and the facing workload results in the desired coloring deposit in the anodic coating of the workload, with appropriate uniformity and intensity of color.

This arrangement is especially adapted for use in establishments having conventional anodizing operations, the coloring step being effected in an elongated tank similar to the tanks used for such operations, and constructed to receive workloads, for treatment, in submerged relation at one or more longitudinal regions parallel to the sides of the tank. Thus for example a workload (which has been previously anodized) consisting of a sheet or sheets of aluminum, or a collection of rods, bars or various shapes or other articles of aluminum (e.g. as carried on a rack), is submerged along a central, lengthwise-extending region of the tank and in facing arrangement on one or each side to an electrode structure consisting of the novel assembly or array of spaced, narrow, metallic elements.

The exact reason for achieving the improved results has not been elucidated, except for some indication that the manner in which the flow of alternating current spreads radially or fanlike from the individual electrode elements toward the workload is such as to promote uniform current density at the surface of the load and to diffuse or counteract localized effects, yet nevertheless unexpectedly providing a large enough flow of current, even though from relatively very small electrode sources, to yield the desired intensity of coloring in reasonably short times of treatment. It is now believed that when large pieces of metallic sheet are used as counterelectrode, there is or can be a relatively intense flow of current, so to speak, at the severed edges of such sheet elements, but is was not readily apparent or obvious that this situation was or could be a cause of various observed difficulties as for example spalling in the case of nickel treatment, or color mismatch among different workloads, or such problem of adjustment or control of the operation as made it hard to get intermediate colors with copper treatment, the last-mentioned problem being found to arise, apparently, from a special voltage criticality in that the voltage range between light and dark tones has seemed very small.

Nevertheless it has been found, indeed by test, that the new procedure and apparatus, which may involve a considerable multiplicity of sharp edges, in the case of parallel narrow strips, or what may be conceived to be an absence of such edges, in the case of parallel round rods or tubes, and which very advantageously allows the actual efiective counterelectrode area to be small relative to the area, measured by dimensions parallel to the workload in which it is distributed, go far to ameliorate or avoid the above difficulties, and thus attain the desired new results, regardless of theoretical considerations.

A very important aspect of the invention resides in the complete process or procedural combination that involves a mode of electrical control which is novel for the described AC coloring step and is cooperatively related to the above concept of employing a multiplicity of counterelectrode elements or surfaces, indeed in being available or feasible, according to present understanding, only when such arrangement or feature respecting the counterelectrode is employed. Specifically it has been discovered that under such circumstances, an unusually effective control of the coloring step is achieved by controlling the electrical current so as to maintain a selected value or values thereof. Although this may conceivably involve a controlled change or variation in the maintained numerical value of current or a timed program of two or more successive current values, a mode of operation presently found of special advantage is to regulate the electrical system so that for a given workload a selected value of current is maintained throughout the interval of treatment. More particularly, the stated electrical control is directed to the current density at the surface of the work, i.e. whether for designed change, program or preferably a single constant value; it will be appreciated that for each workload the actual current value condition, or each such condition, to be maintained is determined upon merely multiplying the total work surface area under treatment by the desired current density, or each such density, as measured for instance in amperes per unit area.

With the operation so controlled, instead of endeavoring to maintain selected voltage conditions, e.g. a single voltage or a program of voltages as heretofore practiced, the results of treatment are exceptionally reliable and reproducible, and indeed can in most cases be such that a given shade or color, in a given metal-containing bath, is attained simply in accordance with the duration of treatment. It appears, for example, that nonuniformity previously encountered among different workloads is probably occasioned by variation in polarization at the counterelectrode; tests in the case of AC treatment with a nickel bath using a nickel electrode have shown mismatch of color between workloads with different surface areas, even though the externally applied voltage and time of operation were the same. The present process, however, involving maintenance of a selected current density, has been found to ameliorate or indeed to obviate these situation of mismatch, permitting ready, determinable attainment of a selected color or shade. Effects of polarization, to the extent they may occur, are automatically compensated, and at least in presently preferred operation, the control for a given color, after setting the current to suit the size of the work, is the time of treatment as stated above.

This feature of current control is specifically made possible by the employment of the above-described, counterelectrode arrangement and operation; only with the latter has it appeared possible to obtain substantially uniform current density or sufficient approach to such uniformity, as to make the value of total current a significant measure of the rate of treatment. Whereas the new mode of counterelectrode operation or arrangement can be employed with the previous voltagegovemed type of control, the novel current-control concept is only realized to a full degree in conjunction with the stated electrode concept or equivalent provision for uniformity of current density at the work.

BRIEF DESCRIPTION OF THE DRAWINGS By way of example and for illustration of the process, certain embodiments of apparatus utilizing the invention are shown in the accompanying drawings, wherein:

FIG. 1 is a simplified view, in perspective, of a tank for the coloring step, with parts broken away, and with accompanying diagram of electrical supply and control;

FIG. 2 is a transverse vertical section as on the plane 2-2, of FIG. 1, including the coloring bath;

FIG. 3 is a schematic plan view of a tank and operation as in FIG. 1, utilizing a different workload;

FIG. 4 is a vertical section similar to FIG. 2, but of apparatus for handling two workloads in parallel;

FIG. 5 is a schematic plan view of the tank and operation of FIG. 4;

FIG. 6 is a partial view, in perspective, of a portion of tank such as in preceding figures, showing electrode-supporting means;

FIG. 7 is a fragmentary vertical section on line 7-7 of FIG. 6; and

FIG. 8 is a fragmentary vertical section on line 8-8 of FIG. 6.

DETAILED DESCRIPTION In the present operations for coloring aluminum, the work is first anodized in a conventional manner to produce an anodic oxide coating, for example of a sort customarily employed for protective or like purposes. While any of a considerable variety of known operations for this purpose may be employed, as with various electrolytes of which aqueous solutions of sulfuric acid, chromic acid, sulfonic acid such as sulfosalicyclic acid, and suitable mixtures of these with other acids or compounds are examples, and while in some cases AC anodizing treatment may be feasible, effective results are obtained by anodizing the work with direct current, as for periods of 20 to 60 minutes, in an aqueous solution of sulfuric acid, e.g. 15 percent acid by weight. The conditions of the anodizing step do not appear to be critical, being selected largely to suit the thickness and other characteristics of anodic coating needed for protective function, the requirements of the subsequent coloring step being satisfied over a considerable range of thicknesses of porous oxide coating on aluminum.

By way of example, and thus assuming that a workload consisting of a sheet or plate of aluminum has first been anodized and rinsed, the same is then submerged in a suitable tank 22 (FIGS. 1 and 2) for the AC treatment to effect a colored deposit in the oxide coating on the sheet, e.g. a colored oxide or equivalent deposit of a selected metal. To that end the bath 23 in the tank 22 may contain ions of the selected metal, as for instance nickel, and the bath may be prepared in appropriate fashion, as set forth in the above-cited U.S. Pat. No. 3,382,160. Specifically, this should be an aqueous acidic solution and may comprise boric acid, nickel sulfate and ammonium sulfate, all in low concentration, and very preferably having a pH value, below neutrality, of at least about 4.

In accordance with the present invention, and with the workload, i.e. sheet 20, supported by and electrically connected to a bus bar or the like 24, for submergence of the sheet 20 along a central region of the tank 22 (which may have an insulating lining 25), a pair of electrode assemblies 26, 27 are disposed inside the tank and in facing relation to the workload. Each of these counterelectrodes may consist of an array of narrow metal electrode strips or like elements 28 carried respectively by and connected to bus bars 30, 31, whereby these electrode strips 28 extend vertically into the bath and are spaced, on each side, along a plane parallel to the work 20.

For electrical energization the bus bars 30, 31 are connected together to one conductor 33 and the bus bar 24 of the work 20 is connected to another conductor 34, these conductors ultimately extending through conductors 35, 36 to receive alternating current from an electrical power source 37. As will be seen from FIGS. 1 and 2, such current is thus passed through the bath to and from the anodized work load 20 from and to the electrode assemblies 26, 27, and as has been explained above, the effect of such treatment, for instance with nickel ions in the bath, is to produce a characteristic colored deposit, presumably nickel oxide, in the anodic coating on the sheet 20. It will be understood that for simplicity, the details of structural support of the metal bus bars 24, 30, 31, which may be constituted of aluminum, copper or other suitable conductive metal, and likewise the details of support of the individual electrode strips 28 are not shown in these views, and indeed may be of any appropriate nature, one form of support for the electrode elements being illustrated in FIGS. 6 to 8.

In further accordance with the invention, the system includes provision for electrical control, to maintain a constant current between the workload and the counterelectrodes, thereby enabling the maintenance of a selected, constant current density at the surface of the workload 20.

Illustrating such operation, FIG. 1 schematically shows current-controlling instrumentalities connected between the conductors 33, 34 and the conductors 35, 36 from the alternating current source 37. Such control means may comprise an ammeter 40 of type suitable for the purpose and a current adjusting means 41, the ammeter having appropriate means (not shown) settable to be effective on departure from a selected current value for adjusting the instrumentality 41 to restore the current in the circuit 33, 34 to the selected value when it has departed therefrom in either direction. Although the device 41 may be of the nature of a variable autotransformer that actually changes the output voltage in the supplied circuit, the control system is nevertheless related to current and functions to maintain the predetermined current as governed by the setting of the meter device 40. Since apparatus of this sort or of other suitability for the described function is known and available, the same need not be shown and described in detail. Indeed in some cases the procedure of maintaining a selected current value can be practiced manually, i.e. as by manual adjustment of the circuit from time to time in accordance with actual reading of an ammeter, it being understood that in many cases the current variations are not so rapid as to preclude such performance.

Inasmuch as the basic aim is to maintain a selected current density at the surface of the work 20, the value of current to be maintained in the circuit 33, 34 is detennined by multiplying the total area of the work, being here both sides together, by the desired current density; the ammeter device 40 is then set to maintain the selected current. Where the work may consist of a plurality of objects, possibly even or irregular shape, as carried on a rack which the central bus bar 24 supports, it is usually sufficient to make an approximate calculation of the total area of such work and of submerged parts of the anodized aluminum rack, for computation of the selected current value. According to present experience and test, good results, for example with nickel or copper baths, can be achieved by simply maintaining a single, selected current density throughout or substantially throughout the time of treatment, it being found that the intensity or shade of color imparted to the workload, up to the darkest color, is then dependent essentially only on such time, i.e. the duration of treatment. The composition of the bath should, of course, be kept substantially constant, and calibration of the process to determine the relation of time to color is a simple matter of test. In consequence with only a determination of approximate area of the work (say, within 10 percent) and setting the control accordingly, colors of successive workloads can be accurately matched by utilizing the same lengths of time.

According to present investigations, an optimum and notable useful value of current density for treatment with nickelcontaining baths, such as hereinbelow exemplified and having a pH of about 4 to 4.5, is about 3 amperes per square foot; more generally, it appears that preferred results are attained under these circumstances by selecting a current density above about 2.5 amp./sq.ft. No advantage has been apparent for operation above about 3.5 amp./sq.ft. or indeed even that high, while substantially larger values may even be deleterious. The basic value of 3 amp./sq.ft. appears to to have reasonable tolerance, in that as mentioned above departures within about 10 percent, as in calculating the area of a given workload, still provide a good color match between successive loads treated for the same length of time. It will now be appreciated that suitable current densities for baths containing other metal ions, of the group stated hereinabove, can readily be determined by an appropriate series of tests in each case. For instance, it has been found that a current density of 5 amp./sq.ft. is effective in the case of copper-containing baths, to permit selection of color or shade in accordance with duration of treatment, and apparently with a tolerance of at least the same proportion as for nickel.

As indicated above, mismatch problems arising when control has been effected by voltage setting or settings may have been caused by polarization effects, e.g. at the counterelectrode especially in that such effects differ with different areas of workload, and possibly also in other ways. The described current control appears to have a more direct relation to the coloring action of the treatment, and coacts in overcoming the problems, apparently in the sense that voltage difficulties due to polarization are automatically compensated. Likewise in situations where voltage control has been extremely delicate, and indeed sometimes impossible to maintain, as for achieving intermediate colors with copper baths, the complete system involving the multiplicity of electrode elements and the described current control, afl'ords optimum results in ameliorating or obviating such difficulty. A setting of current is achieved, to provide a selected or optimum current density, and each shade or type of color is reproducibly obtained by choice of duration of treatment. It is conceived, of course, that in some cases a predetermined variation of current, or a program of current density control may be found desirable, for example two or more successive intervals of mutually different current density values; such mode of operation nevertheless realizes the basic advantages, described above, of using current conditions as the point of control.

Reverting to the drawings, FIGS. 6 to 8 illustrate, in somewhat simplified manner, one way of attaching the electrode strips 28. The wall 42 of the tank 22 has an upper flange 43 to which the bus bar 30 may be secured by appropriate clamps 44. As will be understood, the tank is suitably nonconducting and chemically inert as exposed to the electrolyte; for example, it may be constructed of mild steel or acid-resistant concrete, with an interior insulating lining (not here shown) of suitable resin, plastic or the like, e.g. synthetic rubber or polyvinyl chloride.

The upper ends of the metal electrode strips 28 are fastened to the bus bar, as by bolts and nuts as indicated at 46 in FIG. 7, while the lower ends of the strips extend through suitable openings in an angle member 48 having one of its webs 49 secured to the tank wall. This member is preferably made of electrically and chemically resistant material, e.g. a plastic such as polyvinyl chloride, which permits fastening of the web 49 to the tank lining of like material by welding or fusion. The ends of the strips 28 are held by a suitable pin or key 50 set in an opening in the strip and bearing on the underside of the flange of the angle 48. It will be understood that rod or tubular electrode members can be similarly supported, and may have a nut (not shown) threaded below the flange 48 for similar retaining function. It will also be understood that where a central array of electrode elements, i.e. along the middle of the tank is used, as in FIGS. 4 and described below, a like supporting structure for hanging the strips from the bus bar and carrying their bottom ends by a longitudinal member supported on legs or other means above the tank bottom, may be utilized.

FIG. 3 is a diagrammatic plan view of a tank 22 such as shown in FIG. 1, with strip electrodes 28 along the sides, this figure showing a collection of anodized aluminum objects 52, 53 to be treated, these being supported by an appropriate rack or the like, indicated by the connecting bus bar 24a. In FIG. 3 a number of dotted lines as at 54, 55 have been shown fanning out from the electrodes 28 toward the workpieces 52, 53, as an approximate representation of the manner in which it is understood that current flows between the work and the electrodes, in paths of substantial intensity. While these areas of current travel are probably not in practice as sharply defined as shown, it will be appreciated that the electrode structure thus distributes the current with considerable uniformity over the workload, whether such load comprises separate pieces 52, 53 as in FIG. 3, or a single sheet or plate as in FIGS. 1 and 2. It is usually important, for the desired, superior results, that any load of individual pieces, whether alike or heterogeneous, should be arranged in a regular and advantageously symmetrical fashion on the rack, e.g. as shown, to achieve as even a distribution of electrical current as possible.

FIGS. 4 and 5, the latter being in diagram, show a tank 58, including its insulating lining 59, arranged to accommodate two workloads 60, 61, lengthwise thereof, with an array of metal electrode strips 62 down the middle between the localities occupied by the workloads. Spaced metal electrode strips 28 are provided along the sidewalls, exactly as with the tank 22 of FIG. As will be seen in the diagram of FIG. 5, the spacing of the electrode strips 62 is closer together than the side strips 28. Specifically, where all of the strips are of the same size and thus of the same area on each face, the central strips must deliver current from the respective faces in opposite directions to the workloads 60 and 61. On the other hand, the side strips 28 can be considered as delivering current from both faces, including paths that bend around the strip, for travel to a single opposing face of the workload. Thus in such circumstances, it is found that the spacing of the central elements 62 should be approximately one-half that of the outer elements and that each workload 60 or 61 can then be situated midway between the central and outer electrodes, it being understood that other adjustment for maintaining a uniform, effective current-passing area in the electrode systems, as by employing wider electrode strips along the middle, can be utilized. The design of FIG. 5, with twice as many similar-sized strips in the center, is especially appropriate, tests indicating that with a full load of work 60, 61, the current is practically identical in each of the strips 28, 62, wherever located.

As has been stated, the electrode structures shown afford exceptionally good compensation or correction for the effects, presumably including edge effect, that now appear to have been responsible for some difficulty in certain applications of this electrolytic coloring step. Rods, tubes or like elongated elements or corresponding grids of variously shaped pieces, preferably with large openings, may alternatively constitute the electrode structure. Although indeed still other designs may be employed with considerable corrective function, even a multiplicity of strips or bars rather closely spaced along a vertical plane but nevertheless providing a considerable multiplicity of edges, the open arrangement illustrated is exceptionally simple and satisfactory, with advantages of economy as well as of a mechanical sort and of demonstrated ease of control. For instance, with widely spaced, narrow metal strips forming a central counterelectrode as at 62 in FIGS. 4 and 5, the supporting bus bar 64 can be much smaller in cross section (yet still have ample current capacity) than would be required to support a full sheet or plate electrode; economy of material results and likewise better clearance for insertion of workloads, especially racks of same. The strips, e.g. of nickel or copper as appropriate, are also replaced with unusual ease and convenience when necessary, i.e. when consumed as a result of their function of supplying metal ions into the bath.

In a more general sense, an important feature of all presently contemplated arrangements is that the above-mentioned edge efiect is nullified either by essentially eliminating sharp edges or by maximizing their composite action, e.g. in having so many of them that the radially directed current paths either collectively cover all areas of the work side-byside, or have a multiple overlap, the net result in all cases being substantial uniformity of current density throughout the faces of the workload. Thus the current paths 54, 55 in FIG. 3 show, approximately, how the edges of the individual strips can be considered as one, e.g. each strip 28 being roughly a line source of current, and how the spacing of the strips distributes the current over the work 20. To a significant extent some useful current distribution is attainable with any regular array of sufficiently numerous strip elements, uniform in size and spacing, and regardless of the selected size and the selected spacing within a considerable range of selection. For instance, lO-inch wide strips with a 86-inch space between them can exhibit much of the same current distributing function as %-inch wide strips separated by 10-inch spaces, but of course the widely spaced array of narrow elements has special advantages and appears to afford a specific, exceptionally superior process, as elsewhere herein explained.

The spacing between work and counterelectrode is not especially critical and may conveniently agree with spacing used in conventional anodizing operations, e.g. from 6-inches to 2 feet, a distance of 9 to 12 inches or so affording a convenient balance between shortness of current paths and room to get workloads in and out of the tank. Of course, the number of separate elements of the counterelectrode should be coordinated with the work-electrode distance, at least to insure substantially uniform and complete coverage of the workload region with paths of significant current flow from the electrode. Thus there should preferably be a sufficient number of electrode elements so that their center-to-center distances (in the electrode plane) are about equal to or less than the workload-electrode distance. Some latitude of larger center-tocenter element spacing is possible in some instances, conceivably to as much as 50 percent more than the distance to the work, but for superior results the spacing of element centers should not in most cases be substantially more than about 1 foot, and generally less than 18 inches.

Where advantage is taken of the concept of wide openings between counterelectrode elements (and using correspondingly narrow elements), it is presently conceived that the actual, effective, i.e. current-passing area of the total of an array of elements should preferably be at least about 3 percent, and with special preference at least about 5 percent of the total space or area (measured as one side of it) over which the electrode elements are distributed. In relation to these percentages, the efiective area of an electrode element is generally intended to mean the total surface area from which current significantly flows to and from one facing workload; under such definition for example in FIG. the total effective area of one of the arrays of outside strips 28 is the sum of the areas of both faces of such strips, while the total effective area of the center array of strips 62 relative to a single facing workload is the sum of the areas of one face of each of the latter strips. Although in a broad sense there is no upper limit to the total effective area of the multiplicity of elements, the special advantages of an open counterelectrode assembly would appear to be best realized only when the total, effective, currentpassing area of the assembly relative to a facing workload is not more than 50 percent and with special preference nor more than about 25 percent of the total space, measured as the product of length by height, in which the elements are distributed.

By way of specific example, the AC coloring step was practiced in a 4-foot-wide tank, a size conventional for anodizing and similar operations, using an acidic, nickel-containing, aqueous bath of the following composition, maintained at a pH of about 4 or above, eg 4 to 4.5:

NiSO.7H,O 25 g.p.l. (grams per liter) MgSO,'H,O 20 g.p.l. H BO 25 g.p.l. NH, ,so, l5 g.p.l.

Operation was first effected using counterelectrodes of essentially complete metallic nickel sheets, at the sides of the tank and along the center. To attain a dark bronze shade on workloads, e.g. aluminum sheet which had been anodized to provide a conventional porous oxide coating, alternating current was passed through the bath between work and counterelectrodes, first at l 1 volts for 2 minutes, and then at 17 volts for minutes. The dark tone was reached, but the coating spalled, leaving small or minute bright spots, and other tests indicated that it was practically impossible to obtain more than a medium bronze color without spalling.

The sheet counterelectrodes were replaced with metallic nickel strips each xi-inch wide, arranged vertically as in the drawings, and with reference to FIG. 5, on 9-inch centers along the sides (strips 28) and on 4-l/2-inch centers along the center (strips 62). Operation was controlled to maintain a constant current, specifically to provide a density of 3 amperes per square foot of anodized surface of the workloads, such as anodized aluminum sheet. Under such conditions a highly desirable dark bronze color was obtained after a treatment time of 12 minutes and slightly darker (maximum) after minutes, with no spalling or other defect in either case. Tests also indicated that lighter shades were obtainable, reproducibly, at selected shorter times of treatment.

Advantageous results have also been obtained, as a further example, in operations with copper electrodes using a water solution containing copper sulfate (e.g. 35 g.p.l. CuSo '5H O) and sulfuric acid at about pH 1.3. With counterelectrodes consisting of narrow copper tubing spaced center-to-center by 9 inches, or even 12 inches, a full range of colors from light to black was reproducibly attainable, including medium shades, and when operation of this sort was controlled by current value, e.g. to maintain about 5 amp./sq.ft. on the anodized work, the color reached in each operation was essentially determinable by the time of treatment alone.

While in the foregoing description attention has been given to metal electrode elements, especially nickel elements for nickel baths, copper for copper baths, and the like, as being a specific novel aspect of the invention, it may be noted that similar principles of providing uniform current distribution on the work are applicable to counterelectrodes of carbon, i.e. graphite, which can likewise be employed as widely spaced elements. Moreover, such arrays of graphite rods or bars. governed by substantially the same criteria. provide effective operating conditions for the described novel current control,

i.e. to maintain a selected current density at the surface of the workload and thus to permit determination of the result of the AC coloring treatment by its duration.

It is to be understood that the invention is not limited to the specific materials, operations and structures herein described but may be carried out in other ways without departure from its spirit.

What is claimed is:

1. In a method of producing an inorganically colored anodic coating on an aluminum surface of a workload wherein said surface has first been anodized to produce an anodic coating thereon and wherein alternating current is passed between said workload and an electrode while both are immersed in an aqueous acidic bath containing metal ions for producing a colored deposit in the coating, the procedure comprising passing said alternating current between said anodized surface and only a plurality of electrode elements, of said electrode, that are mutually widely spaced and distributed along a region which faces said workload, said electrode consisting of a multiplicity of said elements distributed with center-to-center distances not greater than about 18 inches and not larger than about 50 percent more than the distance between said region and said workload, and said elements having a total effective surface area for passage of current to said anodized surface equal to at least about 3 percent and not more than about 50 percent of the area of said region in which said elements are distributed, said elements being distributed and arranged for promoting production of said deposit of desired colored character over said anodized surface without local defects.

2. A method as defined in claim 1, which includes controlling said alternating current passing between said anodized surface and said electrode elements to provide a predetermined condition or successive predetermined conditions of current value during the time said current is passed, for causing the current density at said anodized surface to have a preselected maintained value or program of successive preselected maintained values, suitable for producing a range of color shades of said colored deposit depending substantially only on the time of so passing said alternating current between said workload and electrode elements, and effecting said passage of alternating current for a time selected to produce a colored deposit of selected color shade in said coating.

3. A method as defined in claim 1, in which said electrode elements are metal electrode elements having a total efiective surface area for passage of current to said anodized surface equal to not more than about 25 percent of the area of said re gion in which said elements are distributed.

4. A method as defined in claim 3, in which the metal ions in the bath comprise nickel ions and the electrode elements are nickel.

5. A method as defined in claim 4, which includes controlling said alternating current passing between said anodized surface and said electrode elements to provide a predetermined substantially constant current value during the time said current is passed, for maintaining a substantially constant current density at said anodized surface of about 3 amperes per square foot, to produce a colored deposit in said coating, of a shade dependent upon the duration of said time, and effecting said passage of alternating current for a time selected to produce a colored deposit of selected color shade in said coating.

6. A method as defined in claim 1, in which the metal ions in the bath comprise copper ions.

7. A method as defined in claim 6, which includes controlling said alternating current passing between said anodized surface and said electrode elements to provide a predetermined substantially constant current value during the time said current is passed, for maintaining a substantially constant current density as said anodized surface of about 5 amperes per square foot, to produce a colored deposit in said coating, of a shade dependent upon the duration of said time, and effecting said passage of alternating current for a time selected to produce a colored deposit of selected shade in said coating.

8. A method as defined in claim 1, in which said elements are distributed with center-to-center distances not substantially greater than about 12 inches and have a total effective current-passing area equal to not more than about 25 percent of the area of said region in which said elements are distributed.

9. A method as defined in claim 1, in which the metal of said ions is nickel and said alternating current is controlled to maintain a current density of about 3 amperes per square foot at said anodized surface.

10. A method as defined in claim 1, in which the metal of said ions is copper and said alternating current is controlled to maintain a current density of about amperes per square foot at said anodized surface.

11. in a method of producing an inorganically colored anodic coating on an aluminum surface of a workload wherein said surface has first been anodized to produce an anodic coating thereon and wherein alternating current is passed between said workload and a metal electrode while both are immersed in an aqueous acidic bath containing metal ions for producing a colored deposit in the coating, said electrode being of the same metal as said ions, the procedure comprising passing said alternating current between said anodized surface and only a plurality of metal electrode elements, of said electrode, that are mutually spaced and distributed along a region which faces said workload, said elements having a total effective surface area for passage of current to said anodized surface equal to at least about 3 percent of the area of said region in which said elements are distributed, said electrode consisting of a multiplicity of said elements distributed with centerto-center distances not greater than about 18 inches and not substantially greater than the distance between said region and said workload, and said method including controlling the electrical operation of said alternating current treatment by detecting the value of said alternating current and in accordance with said detected value, controlling said current to provide a predetermined substantially constant value thereof during the time said current is passed, for maintaining at said anodized surface a selected current density suitable for producing a range of color shades of said colored deposit depending substantially only on the time of so passing said current, and effecting said passage of alternating current for a time selected to produce a colored deposit of selected color shade in said coating.

12. A method as defined in claim 11, in which said electrode elements are spaced apart by distances which are at least several times the transverse dimension of the effective current-passing surface of each element, said elements having a total effective current-passing area equal to not more than about 25 percent of the area of said region.

13. A method as defined in claim 12, in which the metal of said ions and electrode elements is nickel.

14. A method as defined in claim 12, in which the metal of said ions and electrode elements is copper.

15. In a method of producing an inorganically colored anodic coating on an aluminum surface of a workload wherein said surface has first been anodized to produce an anodic coating thereon and wherein alternating current is passed between said workload and an electrode while both are immersed in an aqueous acidic bath containing metal ions for producing a colored deposit in the coating, the procedure comprising passing said alternating current between said anodized surface and only a plurality of electrode elements, of said electrode, that are mutually spaced and distributed along a region which faces said workload, said electrode consisting of a multiplicity of said elements distributed with center-tocenter distances not greater than about 18 inches and not larger than about 50 percent more than the distance between said region and said workload and said elements having a total effective surface area for passage of current to said anodized surface equal to at least about 3 percent of the area of said region in which said elements are distributed, and controlling the electrical operation of said alternating current treatment by detecting the value of said alternating current and in accordance with said detected value, controlling said alternating current to provide a predetermined condition or successive predetermined conditions of current value, for causing the current density at said anodized surface to have a preselected maintained value or program of successive preselected maintained values, suitable for producing a range of color shades of said colored deposit depending substantially only on the time of so passing said alternating current between said workload and electrode elements, and effecting said passage of alternating current for a time selected to produce a colored deposit of selected color shade in said coating.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 622 471 Dated November l3! 1971 WILLIAM ERNEST COOKE and PAUL JOHN SAJBEN Inventor(s) It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Col 4, line 12, for "situation" read --situations-- Col. 6, line 5, for "or" read --of-- Col. 6, line 26, for "notable" read -notab1y-- Col. 6, line 33, omit "to" (second occurrence) Col. 9, line 14, for "nor" read -not- Col. 9, line 26 (in table) "MgS04.H O" should read --MgSO -7H 0-- Signed and sealed this 16th day of May 1972.

(SEAL) At best EIDXMRD M.FLETCHER,Jh. ROBFRT GOTTSCHALK Atxtesting; Officer Commissioner of Patents 

2. A method as defined in claim 1, which includes controlling said alternating current passing between said anodized surface and said electrode elements to provide a predetermined condition or successive predetermined conditions of current value during the time said current is passed, for causing the current density at said anodized surface to have a preselected maintained value or program of successive preselected maintained values, suitable for producing a range of color shades of said colored deposit depending substantially only on the time of so passing said alternating current between said workload and electrode elements, and effecting said passage of alternating current for a time selected to produce a colored deposit of selected color shade in said coating.
 3. A method as defined in claim 1 in which said electrode elements are metal electrode elements having a total effective surface area for passage of current to said anodized surface equal to not more than about 25 percent of the area of said region in which said elements are distributed.
 4. A method as defined in claim 3, in which the metal ions in the bath comprise nickel ions and the electrode elements are nickel.
 5. A method as defined in claim 4, which includes controlling said alternating current passing between said anodized surface and said electrOde elements to provide a predetermined substantially constant current value during the time said current is passed, for maintaining a substantially constant current density at said anodized surface of about 3 amperes per square foot, to produce a colored deposit in said coating, of a shade dependent upon the duration of said time, and effecting said passage of alternating current for a time selected to produce a colored deposit of selected color shade in said coating.
 6. A method as defined in claim 1, in which the metal ions in the bath comprise copper ions.
 7. A method as defined in claim 6, which includes controlling said alternating current passing between said anodized surface and said electrode elements to provide a predetermined substantially constant current value during the time said current is passed, for maintaining a substantially constant current density at said anodized surface of about 5 amperes per square foot, to produce a colored deposit in said coating, of a shade dependent upon the duration of said time, and effecting said passage of alternating current for a time selected to produce a colored deposit of selected color shade in said coating.
 8. A method as defined in claim 1, in which said elements are distributed with center-to-center distances not substantially greater than about 12 inches and have a total effective current-passing area equal to not more than about 25 percent of the area of said region in which said elements are distributed.
 9. A method as defined in claim 1, in which the metal of said ions is nickel and said alternating current is controlled to maintain a current density of about 3 amperes per square foot at said anodized surface.
 10. A method as defined in claim 1, in which the metal of said ions is copper and said alternating current is controlled to maintain a current density of about 5 amperes per square foot at said anodized surface.
 11. In a method of producing an inorganically colored anodic coating on an aluminum surface of a workload wherein said surface has first been anodized to produce an anodic coating thereon and wherein alternating current is passed between said workload and a metal electrode while both are immersed in an aqueous acidic bath containing metal ions for producing a colored deposit in the coating, said electrode being of the same metal as said ions, the procedure comprising passing said alternating current between said anodized surface and only a plurality of metal electrode elements, of said electrode, that are mutually spaced and distributed along a region which faces said workload, said elements having a total effective surface area for passage of current to said anodized surface equal to at least about 3 percent of the area of said region in which said elements are distributed, said electrode consisting of a multiplicity of said elements distributed with center-to-center distances not greater than about 18 inches and not substantially greater than the distance between said region and said workload, and said method including controlling the electrical operation of said alternating current treatment by detecting the value of said alternating current and in accordance with said detected value, controlling said current to provide a predetermined substantially constant value thereof during the time said current is passed, for maintaining at said anodized surface a selected current density suitable for producing a range of color shades of said colored deposit depending substantially only on the time of so passing said current, and effecting said passage of alternating current for a time selected to produce a colored deposit of selected color shade in said coating.
 12. A method as defined in claim 11, in which said electrode elements are spaced apart by distances which are at least several times the transverse dimension of the effective current-passing surface of each element, said elements having a total effective current-passiNg area equal to not more than about 25 percent of the area of said region.
 13. A method as defined in claim 12, in which the metal of said ions and electrode elements is nickel.
 14. A method as defined in claim 12, in which the metal of said ions and electrode elements is copper.
 15. In a method of producing an inorganically colored anodic coating on an aluminum surface of a workload wherein said surface has first been anodized to produce an anodic coating thereon and wherein alternating current is passed between said workload and an electrode while both are immersed in an aqueous acidic bath containing metal ions for producing a colored deposit in the coating, the procedure comprising passing said alternating current between said anodized surface and only a plurality of electrode elements, of said electrode, that are mutually spaced and distributed along a region which faces said workload, said electrode consisting of a multiplicity of said elements distributed with center-to-center distances not greater than about 18 inches and not larger than about 50 percent more than the distance between said region and said workload and said elements having a total effective surface area for passage of current to said anodized surface equal to at least about 3 percent of the area of said region in which said elements are distributed, and controlling the electrical operation of said alternating current treatment by detecting the value of said alternating current and in accordance with said detected value, controlling said alternating current to provide a predetermined condition or successive predetermined conditions of current value, for causing the current density at said anodized surface to have a preselected maintained value or program of successive preselected maintained values, suitable for producing a range of color shades of said colored deposit depending substantially only on the time of so passing said alternating current between said workload and electrode elements, and effecting said passage of alternating current for a time selected to produce a colored deposit of selected color shade in said coating. 